Anthony Metzer QC, defence counsel for Elisabetta, claimed the former aide was "caught in collateral cross-fire" between Mr Saatchi and Ms Lawson, like the child of divorcing parents.

Metzer questioned why Saatchi had failed to carry out a full investigation into all of his staff after the fraud allegations came to light, claiming Saatchi knew "Lisa (Elisabetta) was a soft underbelly for attacking Ms Lawson".

Asked by defence counsel Karina Arden if she thought she had ever done anything dishonest in relation to the credit card or the company taxi account, Francesca Grillo replied: "No... Absolutely not."

Under cross-examination from prosecutor Jane Carpenter, Grillo described her former employer Ms Lawson as "extremely generous".

She said she had a wardrobe featuring around 10 dresses from the high-end fashion retailer Miu Miu, costing between £300 and "several thousand" pounds.

It was put to the witness that she was the best-dressed of all personal assistants employed by the couple - and she drew a few ripples of laughter from the courtroom when she replied: "I can't help that. It comes in my DNA - I'm Italian.

Francesca Grillo told the court she did not originally mention the alleged drug use in her first statement to police, but did so after photographs of Mr Saatchi and Ms Lawson at Scott's restaurant in Mayfair emerged in the press during the summer.

In the photos she appears to be crying. It shocked me because she said she didn't cry very often in public.

The one that stuck in my mind was the one of him picking her nose. Maybe he found something relating to drugs.

I maybe thought if he didn't know that, he probably didn't know about the authorisation - the allowance - of the signatures (by Ms Lawson, on personal expenditure).

Francesca Grillo, who along with her sister Elisabetta is on trial accused of defrauding Ms Lawson and former husband Charles Saatchi, said she never saw Ms Lawson taking drugs although she did find evidence of drug use on many occasions.

She said she first saw the rolled-up notes at the couple's Shepherd's Bush home, in the kitchen after a party and also in a guest bedroom.

One of her jobs as Ms Lawson's aide was to exchange items between her handbags, and she would find rolled-up banknotes in those.

Asked how often she found the banknotes, Grillo said: "Frequently. Every time I went through her handbag there was some notes. It was very frequent."Asked if she ever raised the issue of drugs, she replied: "No. I didn't think it was my place."